1. The Field of the Invention
The subject matter of the present invention is refractive, transmissive or diffractive optical elements, comprising a ceramic comprising one or more oxides of the type X2O3, which is transmissive for visible light and/or for infrared radiation and which has a cubic crystal structure (in the following disclosure this means a cubic structure that is analogous to that of Y2O3). According to preferred embodiments X is an element selected from the group consisting of Y, Sc, the lanthanides La to Lu, and in particular is Lu, Yb, Gd or La. Also mixtures of the type X2O3 with oxides having a different stoichiometry, such as for example HfO2 and/or ZrO2, are possible, as long as the cubic structure of the ceramic is maintained.
In the following the ceramic is also referred to as an opto-ceramic. According to the present invention, an opto-ceramic (or ceramic) is, as mentioned above, a highly transparent, polycrystalline single-phase material comprising an oxide. Opto-ceramics are to be understood as a particular subgroup of ceramics. “Single phase” means that more than 95% by weight, preferably at least 97% by weight, more preferably 99% by weight, and most preferred 99.5 to 99.9% by weight are in the crystalline form of the intended composition.
The refractive optical elements are suitable for use in mapping optics, for example objectives having reduced chromatic aberrations, in particular with approximately apochromatic mapping behavior. The optical elements made of transparent ceramic may be used in lens systems in combination with lenses of glass, but also with other ceramic lenses, in particular also in digital cameras, mobile phone cameras, in the field of microscopy, microlithography, optical data storage or other applications in the field of consumer or industrial applications.
2. The Related Art
The main aim in the development of mapping optics is to attain sufficient optical quality with a compact optical set-up, which is as light as possible. In particular for applications in the field of digital image detection in electronic apparatuses, such as for example digital cameras, objectives of mobile phones and the like, the mapping optic has to be constructed very small and lightweight. In other words, the total amount of mapping lenses must be minimal. This requires transparent materials with high refractive index and a dispersion which is as low as possible to thus allow the design of very compact mapping optics having approximately apochromatic mapping behavior.
In the case of microscopy, nearly diffraction-limited mapping optics is necessary for the ocular as well as the objective.
In the field of defense, transparent optics are required which have high transmittance in the visible (380 to 800 nm) and also in the infrared spectral range up to 8,000 nm, ideally up to 10,000 nm and, in addition, which are resistant against influences from outside, such as mechanical action, shock, temperature, change of temperature, pressure etc.
For many other technologies the same applies, for example for digital projection and for display techniques. But also in predominantly monochromatic applications, such as the optical storage technologies, compact systems can be realized by means of materials having high refractive index.
At the moment, the development of mapping optics is limited by the optical parameters of the available materials. By the available techniques of glass melting and glass forming, only such kinds of glasses having high quality can be produced which are below a line which approximately passes through the points defined by Abbe number=80/refractive index=1.7 and by Abbe number=10/refractive index=2.0 in an Abbe diagram plotting the refractive index against the Abbe number. This imaginary line is shown in FIG. 2a by a dotted line. In more detail, glasses having a refractive index of between about 1.9 and about 2.2 and an Abbe number in a range of between about 30 and 40 tend to be unstable, so that it is very difficult to produce such glasses in large amounts and with sufficient quality. Also glasses having a refractive index of between about 1.8 and about 2.1 and an Abbe number in a range of between about 30 and 45 tend to be unstable.
The definitions of refractive index (refractive number) nd, Abbe number vd and relative partial dispersion (for example Pg,F) are in principle well known for a person skilled in the art and may be defined in more detail by the means of specialized literature. In the sense of the present invention, the terms are used according to the definitions in “The properties of optical glass”; Hans Bach, Norbert Neuroth (Eds.), Berlin (i.a.): Springer, 1995.—(Schott series on Glass and Glass ceramics: Science, Technology, and Applications; 1); XVII, p. 410-2., corr. print. 1998, XVII, p. 414.
In addition to refractive index and Abbe number, the relative partial dispersion plays an important role in the selection of an optical material. If it is desired to prepare approximately apochromatic optics, materials having approximately the same relative partial dispersion, but a high difference in the Abbe number must be combined. If the partial dispersion Pg,F is plotted against the Abbe number (FIG. 2b), most glasses are on one line (the “normal line”). Therefore materials are desired having behavior with a different combination of Abbe number and relative partial dispersion.
At the moment materials, which are above the before mentioned imaginary line in an Abbe diagram, are exclusively single crystals or polycrystalline materials. However, the production of single crystals by means of the known crystal drawing processes is extremely costly and has enormous limitations with respect to chemical composition. Furthermore for most applications crystals cannot be produced close to the final format, so that this results in an enormous effort of post-processing. Although polycrystalline ceramics can be produced within a broader range of compositions, normally they have insufficient optical qualities, in particular with respect to the homogeneity of the refractive index and the transparency. Till today, only few ranges of compositions and structure types are known, in which transparent ceramics having sufficient optical quality can be produced.
Therefore, polycrystalline ceramics have only been used to a limited extent in optical applications till today. Thus for example, the Japanese Patent Publication JP 2000-203933 discloses production of polycrystalline YAG by means of a special sintering process. Recently, also the production of polycrystalline YAG of optical quality as a laser host material has been achieved, for example for doping with laser-active ions, for example Nd.
In U.S. Pat. No. 6,908,872 translucent ceramics are described which use barium oxide as an oxide, which has to be present in the ceramic. The thus obtained ceramics have a perovskite structure and are para-electric. However, ceramics containing such barium-containing phases having perovskite structure often have insufficient optical mapping quality. This is a result of the tendency of many perovskites to form distorted ferro-electric crystal structures and thus to loose their optical isotropy. Inter alia, this results in undesired double refraction of the crystals, from which the ceramic is built, and, in addition, the transmittance in the range of blue light (about 380 nm) is insufficient.
U.S. Pat. No. 3,640,887 describes production of opto-ceramics on the basis of cubic oxides of the stoichiometry X2O3 (“sesquioxides”). In an exemplary way, only optically active oxides are mentioned which are colored because of absorption bands in the visible (wavelengths of ca. 380 nm to 800 nm). As a sintering aid, i.a., ThO2 is used. This one is not desired due to toxicity, respectively radioactivity. The like belongs to U.S. Pat. No. 3,545,987.
U.S. Pat. No. 4,761,390 discloses a cover plate that substantially consists of a Y2O3 ceramic.
Also U.S. Pat. No. 4,755,492 describes a transparent ceramic Y2O3 as well as its production from powders, which are produced by oxalate precipitation processes. The applications relate to discharge vessels for high-pressure discharge lamps.
U.S. Pat. No. 4,098,612 describe transparent ceramics of mixed oxides of Y2O3 and Al2O3 for discharge vessels. Al2O3 may be contained in an amount of up to 5% by weight, which results in the absence of a cubic structure. The like belongs to transparent ceramic Y2O3 having high contents of La2O3 from U.S. Pat. No. 4,147,744. U.S. Pat. No. 4,571,312 and U.S. Pat. No. 4,747,973 describe opto-ceramics of the system Y2O3—Gd2O3 which, doped with lanthanides that are optically active in the UV-VIS (ultraviolet-visible range), are used as optically active scintillator materials for medical techniques.
JP 2003-128465 and WO 06/03726 describe production of opto-ceramics on the basis of Sc2O3 and Lu2O3. To them are added optically active additives and thus, they are of interest for laser systems.
US 2006/061880 and US 2006/062569 describe the combination of optical mapping systems consisting of at least one lens of ceramic and additional lenticular components of glass, but advantageous effects of the ceramic (owing to e.g. a convenient dispersion behavior) for the whole system is not mentioned. The ceramic lens with extremely high refractive index (nd=2.08) is in direct contact with a glass lens (nd=1.62). Particularly thus costly measures have to be taken to avoid the problem of light scattering associated with the high difference in nd. Thus e.g. in US 2006/062569, the ceramic lens must be connected with a glass lens, the light scattering must be reduced and distributed homogenously over the image detector by a special arrangement of this glass-ceramic-putty member in the optical mapping system.